This invention relates to a code transmission system in which each of a transmitter and a receiver or receivers comprises a buffer memory for temporarily storing codes to be transmitted or codes received from the transmitter.
In some types of code transmission system, use is made of variable length codes. Examples are variable-length differential pulse-code-modulating and interframe coding systems for use in raising the efficiency of television signal transmission. In a transmitter for such a system, information to be transmitted is encoded into information codes at a predetermined sampling rate. For television signals, the sampling rate is linearly proportional to the speed of scanning. In some other cases, fixed length information codes are produced at an irregular rate. In either case, the information codes as produced are irregularly or unevenly distributed with respect to time. It is therefore next to mandatory to temporarily store the information codes in a buffer memory of the transmitter to transmit the information codes and some other indispensable codes at a predetermined transmission or bit rate. In a receiver of the system, the codes transmitted from the transmitter are temporarily stored in a buffer memory of the receiver and subsequently read out thereof to be pertinently decoded. The transmission rate is generally independent of the sampling rate.
In code transmission systems of the type described, it is impossible to carry out pertinent decoding unless the information codes are decoded at the sampling rate. This is because the buffer memory of the receiver otherwise either overflows or underflows. A system of the type described has already been proposed wherein specific codes of a predetermined code pattern, which may be called synchronization codes, are transmitted from the transmitter at a regular interval at least as a part of the other indispensable codes. In television signal transmission, the synchronization codes may be produced at the instants of the field synchronizing signals. Even with this proposed system, the buffer memory of the receiver must have a capacity sufficiently larger than that of the buffer memory of the transmitter in order to prevent the undesirable overflow or underflow which might otherwise occur due to uncertainty of the time at which the decoding begins.